It is well known that a loading unit such as a bucket may be mounted on a boom carried by a loading vehicle such as a front end loader, the boom being mounted for at least vertical movement so that a load material scooped into the bucket at a lower level may be raised to a higher level to, for example, permit the load material to be dumped into a truck. Since the boom moves the bucket along an arcuate path in a vertical plane, the attitude of the bucket relative to the ground or vehicle supporting surface changes. This permits load material in the bucket to spill over the edge of the bucket. Attempts have been made to overcome this problem by providing bucket attitude control systems for automatically tilting the bucket relative to the boom as the boom is raised thus keeping the top of the bucket more nearly parallel to the surface on which the loading vehicle rests.
One prior art bucket attitude control system is operative for all positions of the boom, tilting the front edge of the bucket downwardly relative to the boom as the boom is raised. While this arrangement partially eliminates the problem of load material spill-over, particularly at higher positions of the boom, it does not solve the problem when the boom is in a lower range of positions. The reason is that the front edge of the bucket is normally tilted downwardly in order to scoop up a load, and the attitude control system tilts the front edge further downwardly as the boom is raised so there is a greater tendency for material to spill out of the bucket.
Japanese unexamined U.M. application 63-161952 discloses a loading unit attitude control system have a split flow valve which splits the flow of the fluid forced from a boom cylinder chamber as a boom is raised. One part of the split flow is diverted to a loading unit cylinder to adjust the attitude of the loading unit and the other part is returned to a sump tank. The split flow ratio may be changed depending on the output of a fluid pump so that adjustment of the loading unit attitude is independent of pump output. However, since the split flow ratio is kept constant if the pump output flow remains constant, this system suffers the same disadvantage as the system described above.